In optical systems it is often required to turn a beam of radiation through an angle of ninety degrees. One common method of effecting this is to reflect the radiation using a front surface mirror having a multilayer dielectric reflective coating having alternate layers of high and low refractive index dielectric materials. This method can pose problems when radiation to be reflected is ultraviolet (UV) or deep ultraviolet (DUV) radiation, and when the radiation is plane polarized in (or parallel to) the plane of the reflection, i.e., the plane of incidence of the radiation on the mirror. Radiation polarized in this way is usually referred to as “p” polarized by practitioners of the art. For purposes of this discussion, “UV radiation” refers to radiation having a wavelength between about 200 and 400 nanometers (nm), and DUV radiation refers to radiation having a wavelength of less than about 200 nm.
The problem arises because the reflection of a dielectric multilayer having a given number of layers is less for p-polarized radiation than for unpolarized radiation, and for radiation polarized perpendicular to the incidence plane (“s” polarized). More layers are required to provide a desired reflection for p-polarized radiation than are required to provide the same reflection for unpolarized radiation or s-polarized radiation. This problem is worse for ultraviolet radiation than for visible radiation. This is because UV transmitting materials typically have a refractive index no greater than about 2.0, whereas visible and infrared transmitting materials having a refractive index of 2.35 or greater are available. The problem is even worse for DUV radiation as materials transparent to DUV radiation typically have a refractive index no greater than 1.75. There are low refractive index materials (materials having a refractive index of 1.5 or less), such as magnesium fluoride (MgF2) or silicon dioxide (SiO2) that transmit both UV, DUV, and visible radiation.
The relatively low maximum refractive index value of UV and DUV transmitting dielectric materials means that more layers are required to provide a given reflection value than would be required to provide the same reflection for visible radiation. By way of example, in order to provide a reflectivity of 99% for p-polarized radiation at a wavelength of 193 nanometers (nm) and at 45° incidence, about 71 layers would be necessary. Only about 15 layers would be necessary to provide the same reflectivity at 45° incidence for p-polarized radiation having a wavelength of about 525 nm.
Internal stress, electronic absorption, and defect content in a multilayer dielectric coating can increase with the number of layers in the coating. Susceptibility to radiation damage by non-propagating pit formation, for example, can increase with increasing stress, electronic absorption or the number of defects in a coating. However, any optically coated surface can usually be expected to be more susceptible to radiation damage than an uncoated surface. Accordingly, there is a need for a method for providing 90-degree reflection of p-polarized radiation, in particular for UV radiation, and more particularly for DUV radiation that does not require the use of a multilayer reflective coating, and preferably does not require any optical coating.